The type of container with which the present invention is particularly concerned is the type made of paperboard stock coated on both sides with a thermoplastic material such as polyethylene. The thermoplastic coating is utilized not only as moisture proofing material, but also serves as an adhesive which cooperates in sealing the seams, joints and closure elements of the container so as to make the container fluid tight when it is filled with milk or other contents, and sealed and closed. An example of this general type of container is disclosed in U.S. Pat. Nos. 3,120,089, 3,309,841 and 3,910,014.
Until relatively recently, the conventional manner of sealing thermoplastic coated paperboard cartons was to activate the thermoplastic coating, by the application of heat, to cause it to flow at the portions to be joined and sealed, and then bring the layers of the end closures into contact with each other. When the thermoplastic material between the layers cooled and was set, the layers were sealed and fused together. In addition to requiring the use of elaborate and expensive apparatus for generating the considerable amount of energy required, elaborate apparatus, methods and systems were also required to dissipate the heat from the plant in which the packaging operation was being carried out. Examples of this general type of packaging apparatus are shown in U.S. Pat. Nos. 3,002,328; 3,120,089; 3,166,994; 3,187,647; 3,239,995 and 3,309,841.
U.S. Pat. Nos. 3,905,280 of Sept. 16, 1975, 3,910,014 of Oct. 7, 1975 and U.S. Pat. No. 3,956,046 of May 11, 1976 disclose apparatus for sealing the end closures and side seams of thermoplastic coated paperboard containers by vibration welding wherein mechanical energy is converted into localized heat energy to minimize the amount of heat and energy required, and to eliminate the necessity for expensive systems and methods for dissipating the excess heat. U.S. Pat. No. 3,526,792 discloses an ultrasonic transducer having a converter for converting electrical energy into mechanical energy, and a concentrating horn for concentrating the mechanical energy into vibrations at a desired frequency. In the above mentioned U.S. Pat. Nos. 3,905,280, 3,910,014 and 3,956,046 the horn vibrates at a predetermined frequency when energized, and cooperates with an anvil or other backup means to clamp the layers of the thermoplastic coated paperboard between the working surface of the horn and backup or work surface of the anvil. When the thermoplastic coated paperboard layers are clamped between the work surfaces of the horn and anvil, vibration of the horn creates frictional heat sufficient to activate the thermoplastic coating of the layers to cause the thermoplastic material of the layers to flow together. When the vibration is terminated, the thermoplastic will cool and set to form a seal between the layers thereby bonding the layers together throughout the area of the seal.
An additional advantage of sealing by sonic, or vibration welding, is that it is not necessary to allow for or eliminate any surface contamination or moisture because of the concentrated application of energy. When heat is applied directly rather than by vibration welding, energy must be expended to evaporate any moisture on the sealing surfaces before the thermoplastic material can be heated sufficiently to activate it. Variations in the amount of moisture on the sealing surface causes a wide variation in the thermal energy required. Moreover, the presence of moisture causes the thermal energy input demand to be far in excess of that which is required to activate the thermoplastic when the surfaces are dry. This problem is obviated by vibration welding because of the concentrated application of energy, making the amount of energy required to evaporate any moisture relatively insignificant.
There are two stages in the sealing operation of thermoplastic coated paperboard containers by vibration welding. These stages are: (1) the activation stage in which the friction of the vibrating horn generates heat sufficient to activate the thermoplastic coating; and (2) the cooling stage wherein the layers are clamped together to permit the previously activated thermoplastic coating to cool and set to form the seal.
A significant factor in determining the production rate is the amount of time required in the sealing operation. It has been a general practice to utilize the horn exclusively for both the activation stage and as a clamping member for the cooling stage. When the horn is used exclusively for both stages, a complete cycle of forming a seal between the layers of the carton involves (1) bringing the working surfaces of the anvil (or other backup means) and horn into opposed relationship on opposite sides of the layers to be joined and sealed, (2) activating the thermoplastic coating of the layers between the work surface by the frictional heat of the energized horn to cause the coating of the layers to flow together, (3) permitting the previously activated thermoplastic material to cool and set while still held between the working surfaces of the anvil and deenergized horn, and (4) separating the working surfaces of the anvil and horn to remove the carton layers after the seal has been formed. The time involved per cycle to date has been in the range of approximately one-half to two seconds, or more, depending upon the thickness of the paperboard, the thickness of the thermoplastic coating, the number of layers, and the surface area to be sealed.
It has been conventional in vibration welding operations of this type to pressure actuate the horn to start the horn vibrating. In some applications the working surface of the horn moves into contact with the layers of the carton to be joined together, the horn starts vibrating when a predetermined pressure on the working surface of the horn is sensed by horn actuating controls. In a high production packaging system, the available time for the sealing operation is determined primarily by the speed at which the cartons must pass through the welding cycle as determined by the speed of other machines in the total system. For example, the welding apparatus must be incorporated into a system including machines for feeding and erecting cartons from flat blank form, such as machines of the type disclosed in Allen U.S. Pat. No. 3,599,541 of Aug. 17, 1971 and in Kellogg U.S. Pat. No. 3,937,131 of Feb. 10, 1976, and filling and closing apparatus such as disclosed in Braun U.S. Pat. No. 3,910,014.
U.S. application Ser. No. 652,916 of Eugene R. Bosche and Earle W. Walke, Jr., filed Jan. 28, 1976, and assigned to the assignee of this application, discloses an apparatus and method for vibration welding thermoplastic coated paperboard containers wherein the vibration welding horn is "pre-triggered" to start it into vibration prior to the time it engages the layers of the container to be sealed. U.S. application Ser. No. 690,221 of Eugene R. Bosche, filed May 26, 1976, and assigned to the assignee of this application, discloses a method and apparatus for forming a seal between the layers of thermoplastic coated paperboard cartons by vibration or sonic welding in which the vibration welding horn is utilized to activate the thermoplastic material of the carton. The carton is then moved to a clamping and cooling station with the thermoplastic material still activated, where the activated layers are then clamped and cooled to form a seal. The entire disclosure of U.S. application Ser. Nos. 652,916 and 690,221 are incorporated herein by reference.
In U.S. application Ser. No. 846,481 the concept of activating the thermoplastic coating at one station and cooling and clamping the layers at another station (i.e. two-station sonic sealing) is further developed. It has been found desirable, at least in some circumstances, to permit partial setting of the activated thermoplastic at the activation station. The horn is pre-triggered to cause it to start vibrating before it comes into clamping engagement with the paperboard layers. After the activation time has been completed, the horn stops vibrating but remains in clamping engagement with the layers to permit partial setting of the activated thermoplastic to begin formation of the seal. The horn is then withdrawn, and the container moves to a clamping and cooling station to complete the formation of the seal by clamping and cooling the layers for the time necessary to complete the setting of the activated thermoplastic.